As far as I know, none of my disc friends are collectors, so I don't have anyone to ask, so would someone take a moment to explain the designations and reason for collection?

What's the meaning of the number x designations, such as 3x, 11x?
Do the numbers have meaning or are they just a designation?
Where on the disc does one find the markings?
Are the markings on all discs or just some?

And lastly, why do you collect, is there a market in rare discs or is it just to satisfy the drive to have something unique?